Position measurement by laser beam

ABSTRACT

A laser beam from a source 20 is used to provide a reference in space adjacent a portion of a structure movements of which are monitored. The beam is received by a receiver 30. A number of targets 300 may be mounted to move with parts of the structure extending alongside the beam path. Target constructions (FIGS. 6-8) are described which allow for normal passage of the beam past the target but which enable the position of the beam relative to a given target to be measured by intercepting the beam. In a variation (FIG. 13) for civil engineering use, a single target is selectively mountable to one of a number of supports. Another variation (FIG. 11) uses graduated mesh targets on which the beam impinges on each mesh but is also transmitted through the mesh. Dynamic movement of a part of structure such as a bridge is measured with a single target attached to the part in question and comprising a barred reflective assembly (FIG. 3), a fibre optic array (FIG. 4) or a position sensing photo-diode (FIG. 5). For use in such systems there is also described a spatial filter for a laser receiver (FIG. 10), a laser assembly adapted for precise repositioning (FIG. 9) and a dust excluding laser housing (FIG. 12).

FIELD OF THE INVENTION

This invention is generally concerned with the use of a laser beam toprovide a reference in space against which movement of or withinstructures may be measured or parts may be aligned. The structures maybe natural or man-made.

In buildings, there often arises the need to measure either absolutely,or with respect to preset limits, movement of a part of the building,e.g. settling movement. In structures such as bridges, it may bedesirable to measure the dynamic response of the structure to loads onit, e.g. the deflection of a bridge in response to loads placed on it.For some natural structures there is a need to monitor ground movementsor differential displacements within the structure. In this connectionmonitoring in earthquake prone areas comes to mind. Thus there are twobroad types of movement. One is progressive movement, essentiallywithout recovery: the other is dynamic movement with recovery. In othercases there may be a need to bring two separated parts of a structure orapparatus into a required alignment.

BACKGROUND OF THE INVENTION

There have been various proposals for using a laser beam as a referencethat impinges on a target which is mounted to a structure, or a part ofa structure under surveillance. As the structure moves the target moveswith it. The beam provides a spatial reference against whichdisplacements of the target are detectable. It is, or course, possibleto mount the laser source to whatever is to be monitored. Normally,however, the target will be fixed to the surveyed structure liable tomovement and the laser source mounted to a fixed point.

Many of the proposals provide for a laser source to direct its beam at atarget which, for example, comprises some form of photocell assemblywhereby the position of the beam on the assembly can be detected.Examples of this type can be found in British Patent Specifications Nos.1,178,007: 1,313,416 (in which refractive displacement of the beam ismeasured): 1,323,104 (which is concerned specifically with buildingstructures): 1,338,167 (which is also concerned with alignment inbuilding structures): 1,372,145: 1,436,740 (which is for contourmeasurement as the target is moved along a path): 1,513,380: and in PCTpublication No. W081/03698. Except for specification 1,372,145, all theabove-mentioned specifications essentially measure a relative laterialdisplacement between the laser source, which is usually taken to bemounted on a location taken as fixed, and the target which is on thepart subject to displacement. Specification No. 1,372,145 (U.S. Pat. No.3,799,674 to Guillet et al) shows monitoring of a dam in which apparentdeflections of the beam relative to two targets mounted on the dam wallare detected in respective orthogonal directions by respective targets.It will be appreciated that in general the optical system should be keptas simple as possible to avoid unwanted displacements due to relativemovement of parts of the system, such as arising from temperaturechanges.

There has arisen a need to be able to monitor structures at variouspoints for signs of movement with apparatus that requires littleattention and supervision. One system for monitoring an elongatestructure at various points along its length is disclosed in Britishpatent specification No. 2,101,305 in which displacements in the hull ofa tanker are monitored by directing a laser beam along the tanker andlocating a reflecting target at points along its length. Measurementsare made by interferometry between the reflected beam and the primarybeam. In this case one reflector target point at a time is measured. Thesystem is relatively complicated to set up and relatively complexprocessing circuitry is required. An older technique for use insurveying railway tracks is the so-called shadow board technique such asdescribed in British patent specification No. 1,322,785. Here again onlyone point along the track is measured at a time.

SUMMARY OF THE INVENTION

A first aspect of the invention provides for monitoring whether thelaser beam as a datum has itself moved by also monitoring the positionsof one or more reference laser beams in fixed spatial relationship withthe first mentioned (primary) beam. The reference beam or beams could bederived from the primary beam but other ways of deriving thereference(s) are presently preferred and form second aspects of theinvention.

A third aspect of the invention lies in providing a measurement ofactual movement of a target mounted on a surveilled structure andtargets suitable for that purpose. Such arrangements find particularapplication in dynamic measurements as will be described. Targetconstructions are described to provide a measurement along a single axisor along two axes (X-Y). Some target constructions effectively allowonly one target to be used. These target arrangements include novelarrays of detector elements and novel patterned mirrors.

A fourth aspect of the invention relates to plural targets that areusable to track the beam along at least one axis, the tracking alsoproviding a measure of beam position relative to target. Variousconstructions will be described.

A fifth aspect of the invention lies in the provision of means for alaser to prevent contamination of the optics and consequent loss ofperformance.

A sixth aspect of the invention lies in a system for monitoringmovements of a structure in which a number of points in the structureare provided with support means that move locally with the structure. Atarget is detachably mounted to a selected one of the support means,which are adapted to provide a repeatable positioning of the target onthe support means, whereby a local measurement of the target relative toa laser beam reference may be made.

The invention and its various aspects will now be further described withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, 1D and 1E show a general view of a monitoring systemas disclosed in commonly assigned U.S. application Ser. No. 623,060filed on June 21, 1984 and now abandoned, for a structure usingapertured targets (detectors), FIGS. 1A, 1B, 1C and 1D showing detailsof the system, and FIG. 1E a receiver block diagram;

FIGS. 2 and 2A show a modification of the system of FIG. 1 with a lasertransmitter arrangement between two receivers for monitoring astructure, which laser transmitter arrangement may find use in otherapparatus subsequently described;

FIGS. 3, 3A, 3B and 3C show a system embodying the invention for makingactual measurements of deflection of a structure, particularly underdynamic conditions, FIGS. 3A and 3B illustrate patterned targets, andFIG. 3C shows a detector output signal;

FIGS. 4, 4A and 4B show a modification of the system of FIG. 3 to use atarget comprising a matrix of detector elements seen in FIG. 4A and toan enlarged scale in FIG. 4B;

FIGS. 5 and 5A show another version of the system of FIGS. 3 and 4 butusing another form of detector element in the target, and FIG. 5A showsthe detctor element and its associated circuitry in more detail;

FIGS. 6, 6A and 6B show a development of the system of FIGS. 1 or 2which is in accord with this invention and in which a plurality ofposition measuring targets are usable along a laser beam datum toprovide individual beam position (X-Y) measurements, FIG. 6A showing onesuch target and FIG. 6B details of the target;

FIGS. 7, 7A and 7B show another development of the system of FIGS. 1 or2 which is in accord with the invention and in which a targetconstructions is shown that enables the target to track the beam, i.e.to remain aligned with it, FIGS. 7A and 7B showing face and partialsection views of one target;

FIGS. 8A, 8B and 8C show a modified construction of a beam trackingtarget, and also illustrate means for measuring target and thusposition, such means being applicable also to the target of FIGS. 7A and7B;

FIG. 9 shows a laser assembly providing plural reference beams andassociated beam position measurement means for use in aligning the lasertransmitter unit;

FIGS. 10A, 10B and 10C ilustrate a receiver with a spatial filterapplied to a convergent and divergent laser beam respectively, atechnique of general application in various of the embodiments hereindescribed;

FIGS. 11, 11A, 11B, and 11C show another system of the invention formonitoring progressive movement, FIG. 11A showing one of the targets,FIGS. 11B a side view of a target housing, and FIG. 11C a detail of thetarget in operation;

FIG. 12 shows a laser provided with an air purging device to preventcontamination of the optics in accord with this invention, and

FIGS. 13, 13A, 13B and 13C show another system according to theinvention applied to an embankment or like structure, FIG. 13A showingone mounting point for a target, FIG. 13B showing a simplified face viewof a target, and FIG. 13C showing a detail of the target.

In order that apparatus and systems embodying the invention anddescribed subsequently may be better understood, there will first bedescribed with reference to FIG. 1 a system as disclosed in the Ser. No.623,060 application, and which serves to illustrate certain underlyingprinciples of this invention.

Referring to FIG. 1, there is shown in diagrammatic form a system usinga laser beam as a spatial referenc to monitor movement within a buildingstructure generally indicated at 10.

At a point 12 of the structure that provides a reference location thereis mounted a low power laser 20 supported by a cradle 22 secured to thestructure in any appropriate fashion. The laser 20 is mounted forpivotal movement in a vertical plane about axis 24 and the cradle has atleast some degree of azimuth adjustment of the laser 20 between parts26A and 26B. In addition the cradle assembly should allow, for exampleby the use of slotted guides, for some absolute adjustment of positionin the vertical and horizontal planes. This combination of linear andangular adjustment allows for accurate alignment. Once aligned theassembly is securely locked in position by means not shown. The laser 20and cradle 22 are shown to a larger scale in FIG. 1A.

The laser 20 has an optical system 28 which preferably includes an iris(not shown) for assisting in limiting beam divergence. At the other end13 of the optical path 40 is mounted a receiver 30 supported by a cradle22' of the same construction as the laser cradle 22 to allow alignmentof the receiver. The receiver includes input optics 32 which arediscussed further below. The receiver 30 and cradle 22' are shown to alarger scale in FIG. 1B. The laser and receiver are aligned on the laserbeam axis 40 which inpinges on one or preferably a succession of targetsor movement detectors 42 located along the beam path. Each target 42comprises a support stem 43 mounted to a portion of structure 10 to bemonitored; a depending plate 44 having a relatively large aperture 46therethrough; and a target plate 48 having a smaller aperture 50 alignedcentrally with that in plate 44. FIGS. 1C and 1D show views of oppositesides of a target. The plate 48 is adjustably mounted by any suitablesupport means 52 to the plate 44 so that the target plate aperture 50 isalignable with the beam, i.e. the beam impinges on the aperture 50 so asto pass through the target.

The receiver shown in block diagram in FIG. 1E comprises aphoto-detector 60 for the laser beam, a threshold circuit 62 whichprovides an output when the received light level drops below anadjustable predetermined value and a timer circuit 64 providing a delayso as to only provide an output indicative of movement in the structurewhen the threshold circuit output is maintained for at least apredetermined period. This is to avoid warning outputs being given formomentary interruptions of the beam. Movement of any target 42 by mroethan a predetermined amount determined by the size of aperture 50 willcause the received light to drop to a value low enough to activate thethreshold circuit 62.

The system shown in FIG. 1 shows a straight-line optical path. It willbe appreciated that with the use of mirrors the path can be configuredas desired. The apertures 50 would normally be circular though othershapes are possible to define different allowable movements in differentdirections. The laser can operate at wavelengths within the visiblelight spectrum or outside. Visible light has an advantage that the beamcan be seen on a target for alignment purposes.

Referring now to FIG. 2 there is shown a modification of the arrangementof FIG. 1 in which a laser transmitter 120 is mounted between tworeceivers 30 with interposed targets on respective beam paths 140. Tothis end the laser transmitter 120 is either constructed as adouble-ended laser or as a pair of lasers rigidly mounted back to back.The double-ended laser is contemplated to be realised by having partialmirrors at both ends of the optical cavity to allow the laser light toemerge in opposite directions, as seen in FIG. 2A.

The arrangement of FIG. 2 has the advantage of making possible a longertotal surveillance path. For safety reasons the power of a laser beam isrestricted thereby restricting the length of the path over which auseful signal is receivable. Thus the centrally mounted lasertransmitter enables a total path length double that otherwisesurveillable. A second advantage can be seen by looking at the system ofFIG. 1 or one half of the system of FIG. 2. Misalignment arising betweentransmitter and receiver after installation, by skewing of thetransmitter will be liable to cause a false warning of movement of atarget. In FIG. 2, the two-path arrangement will cause both receivers tooutput a warning if the transmitter skews. Actuation of both receiversin this way would not normally be expected. Thus by looking at bothoutputs together, simultaneous outputs would be taken to be a systemfault, and not due to target movement.

To perform in this way the two beams from transmitter 120 must bemaintained in accurate relationship--not necessarily in directlyopposite directions--by the inherent nature of the laser used in thedouble-ended case or by ensuring a rigid mounting between the laserswhere a pair are used.

The systems of FIGS. 1 and 2 are designed to detect movements or theresult of a succession of progressive movements. By sizing and shapingthe target apertures some variation of the limits along different axeswould be possible. In some circumstances it is necessary to makemeasurements of the dynamic response of a structure under stress, e.g. abridge as mentioned above. In such cases it is necessary to obtain anactual measurement of the value of deflection of movement. There willnow be described embodiments in accordance with the invention forperforming dynamic measurements.

Referring to FIG. 3, there is shown a portion 210 of a structure whichis exemplified by deflecting in a vertical direction under load.Securely mounted to and depending from structure 210 is a reflectivetarget 220 that moves with the structure. At a remove point, e.g. groundproviding a stable positional reference, a laser 230 is mounted havingits beam focused by appropriate optics on the target. The reflected beamis returned to a receiver 240 incorporating a photo-detector. Thereceiver 240 is conveniently, but not necessarily, adjacent, the laser230. It will be appreciated that the figure is diagrammatic and not toscale, so that angles are not shown correctly.

The reflective target is a mirror having a pattern of reflective andnon-reflective zones which, in the illustrated embodiment, are barsextending normal to the direction of motion and each bar having itswidth uniform in that direction. FIG. 3A shows a face view of the mirror222 suspended on supports 224.

The mirror is preferably provided with some weather protection for itssurface, such as a shroud housing. For example one side of the housingillustrated in FIG. 11B to be described. In addition the mirrorstructure may be provided with a fan or blower to maintain a purgingcurrent of air across the mirror surface. This will assist in preventingthe accumulation of dirt on the mirror. The mirror may be provided witha heater or the purging air heated to prevent misting.

In use of the system of FIG. 3, the focused laser beam will effectivelyscan the mirror pattern as the mirror moves vertically with movements ofthe structure. The photo-detector circuit in the receiver 240 willproduce a pulse output as indicated at FIG. 3C as the bars are scanned,the pulse duration and interval being a function of the rate ofmovement, and the number of pulses in a given direction of movementbeing a measure of deflection which is calculable from knowing thepattern dimensions. The pulse output from the receiver can be used byfirst storing the information carried by the pulses and then analyzingthe information. For example the pulse output from the receiver 240could be applied to a variable persistence or storage oscilloscope 250and stored therein.

It is preferred to provide some means for clearly distinguishing thedirection of movement of the target, i.e. up or down in the embodimentillustrated. The target modification illustrated in FIG. 3B providesthis facility. FIG. 3B shows a mirror 222' divided into two parts 223Aand 223B (or two separate mirrors rigidly mounted together) along anaxis (vertical) in the direction of target movement. The mirror partsare in turn divided on a transverse (horizontal) axis into a barred anda non-barred portion 224A, 225A and 224B, 225B respectively. Each barredportion is arranged in reflective and non-reflective bars as describedwith reference to FIG. 3A. The non-barred portions 225B and 224B couldbe non-reflective but are preferably plain mirror areas. As seen fromthe figure the barred areas extend in opposite directions from thehorizontal axis. The laser transmitter requires two lasers 230 (or theuse of beam splitting optics) which are focused at a respective point226A, B just at the edge of a respective barred area 224A, B. Targetmotion in one direction causes the relevant reflected beam to be pulsedfrom one mirror part: movement in the other direction causes the otherreflected beam to be pulsed from the other part. Apart from twotransmitted laser beams, two receivers 240 will be required, one foreach beam.

The two barred areas could overlap in the direction of movement. Themore crucial feature is to ensure that the respective laser beam is setup at the edge bounding the unbarred portion. The advantage of havingthis portion reflective is that movement of the target causing theassociated beam to fall on this unbarred portion will retain a signal atthe associated receiver as a check on correct operation.

Another dynamic measurement system is shown in FIG. 4 in which thestructure 210 has mounted to it a target 220' on which is focused thelaser beam from laser 230 as in FIG. 3. In this case the target ordetector 220' comprises a matrix 260 of fibre optic strands better seenin the face and enlarged face views of FIGS. 4A and 4B. The matrix canbe made by packing a multistrand fibre optic into a bundle which is setin resin and sheared to provide a smooth face with the fibre optic ends262 set on a regular co-ordinate grid. The bundle 264 of fibre opticsruns to a convenient measurement point where it is likewise displayed ina co-ordinate grid corresponding to that of FIG. 4B as shown by unit266. Conveniently in the display unit 266 the fibre optic ends arespaced ou more than at the detector (that is a larger scale grid) tomake for easier analysis. This arrangement makes for direct measure ofboth vertical and horizontal deflection components of the structure 210.For automatic analysis of dynamic responses, the display unit 266 can beprovided with an array of photo-detectors, one for each fibre optic endwhich are scannable to obtain X-Y measurements or are connectable to achart recorder or oscilloscope to plot the deflection as the laser beamimpinges on different fibre optic ends in the target 220'.

An addition to the system of FIG. 4 is to have a second laser directingits beam on the target 220'. The second laser is mounted separately fromthe first and its signal is detected separately, for example byalternately pulsing the two lasers. A movement of the target will causea relative movement of the two laser beams over the target in unison. Ifa relative movement of only one beam is sensed, this indicates that theattitude of the relevant laser has changed rather than there being atrue target movement.

FIG. 5 shows another variant in which the target 220" is made aposition-sensing photo-diode 290--seen in the inset. The diode issegmented and from it are obtained signals representing the displacementof the laser beam focused on the diode from the centre of the diode. Thesignals are sent over cable 270 to an analyzer unit 280 from which thehorizontal and vertical deflection components are obtained. The mannerin which this done is shown in the block diagram of FIG. 5A in which theposition-sensing diode is shown symbollically with a common anode andfour cathodes. The photo-diode device 290 has a centrally placed commonanode 290A and four cathodes 290C1-290C4. The device 290 is oriented inuse such that the opposite cathodes 290C1 and 290C3 lie on thehorizontal axis to provide a pair of X-axis signals CX1 and CX2. Theother pair of opposite cathodes 290C2 and 290C4 thus lie on the Y-axisand provide a pair of signals CY1 and CY2. The magnitude of each of thecathode signals will depend on the position of the laser beam on thephoto-diode relative to the centre. Such diodes are available fromUnited Detectors Technology, Inc. of Santa Monica, Calif., United Statesof America.

The diode is connected to circuitry in the analyzer 280 indicated by thedash line. This circuitry is illustrated to the extent of showing howthe beam position signals are derived. The diode is biased in the usualway with the anode going to a negative rail, each of the cathode signalsbecoming more negative as more light impinges on the relevant segment ofthe diode. Each of the signals CX1, 2 and CY1, 2 is first applied to oneinput (the same sign in each case) of a respective differentialamplifier 292A-D whose other input is connected to the common negativesupply. The outputs of the differential amplifiers 292A and B whichreceive signals CX1 and CX2 are applied to the respective inputs of anX-axis differential amplifier 294 from which is obtained a signal (XSHIFT) whose magnitude and polarity represents the position of the laserbeam on the X-axis. Similarly the outputs of differential amplifier 292Cand D which receives signals CY1 and CY2 are applied to the resepctiveinputs of a Y-axis differential amplifier 296 to produce a signal (YSHIFT) whose magnitude and polarity represents the position of the laserbeam on the Y-axis.

The analyzer may also include further circuitry 298 for indicatingexcursions of the X or Y signals outside set limits--perhaps only thisneed be known--or for combining the two to detect the total radialexcursion from the centre of the target.

It will be appreciated that the arrangements of FIGS. 3 to 5 could beused for sensing movements beyond a set limit as described withreference to FIGS. 1 and 2. However the targets or detectors of FIGS. 3to 5 entirely intercept the laser beam or beams and thus it is notpossible to measure structural movements at a plurality of positionalong the beam.

FIG. 6 shows a system for monitoring the position of the beam withrespect to one or more targets. This system is particularly intended formonitoring structures on a periodic basis over a long term or forcontinuous monitoring. In the system of FIG. 6, there is shown anarrangment having a similarity with that of FIG. 1 in that there is astructure 10, which is shown horizontal but may be vertical, at areference location of which is mounted a laser 20 supported by cradle22. The laser beam is received by a receiver 30 having traversed aplurality of targets or detectors 300 which are mounted along thestructure similarly to targets 42. However, the targets 300 of thisembodiment are constructed to both allow passage of the beam and toprovide an actual position measurement. The principle of operation ofeach target 300 can be seen from FIG. 6A which shows laser beam 310normal to the plane of the figure. The target has a frame 312 in whichare supported two members 314 and 316 providing straight edges 315 and317 respectively movable in orthogonal directions (X-Y). Each straightedge member is drivable across the beam path by a respective solenoid orair/fluid driven actuator 318 and 320 so as to intercept the beam. Bydetecting the movement and/or point of interception, the position of thebeam relative to the target is known.

Normally the succession of targets along the beam would be in the stateshown in FIG. 6A, that is to say with the straight edge members out ofthe beam path. By appropriate successive actuation of the straight edgemembers of the targets such that only one target is active at a time thebeam position on the X or Y axis is measured for one target withoutinterference from another. Considering in more detail how themeasurement at any one target is performed one arrangement isillustrated in FIG. 6B showing one straight edge member, 316 say, andits actuator 320. One end 322 of the straight edge traverses a scale 323of alternate reflective/non-reflective bars. An opto-electronic device,i.e. a photo-emitter and photo-detector device, is mounted on the endportion 322 to illuminate and register optical pulses as the actuatordrives the member 316 across the beam. The pulses are applied to acounter (not shown). As the straight edge 317 intercepts the beam 310,as indicated in FIG. 6B, the receiver 30 output goes low and is used tointerrupt the counter to give a count value that is a measure of beamposition in the Y-direction. A similar arrangement is provided on theX-axis. Because of the successive sampling of the beam by the targets300, a single counter or a single respective X and Y counter can be usedwith consequent economy of circuitry.

The detection of beam interception can be performed by extending anoptical fibre (or light guide) along the straight edge, in fact usingthis to define the straight edge so that as the beam is intercepted,light enters the fibre and is used to activate a detector at one end ofthe optical fibre. The detector output stops the counter in this case.The receiver 30 no longer plays a direct part in the positionmeasurement. While it could be omitted its presence provides a directmeans of monitoring that the laser beam axis is still on its properalignment. Skewing of the laser would eventually cause the receiveroutput to be lost thereby indicating the system was in need ofre-alignment.

The target 300 thus far described requires two orthogonal straightedges. X and Y measurements can be simultaneously made with a singlestraight edge member. Looking again at FIG. 6B the beam 310 isintercepted by the straight edge 317 at a point along its length that isa measure of the X-position. Consequently the straight edge couldcomprise a linear array of photo-diodes which would be AND-gated to stopthe counter for the Y measaurement upon one of them detecting the beam.The individual diode intercepting the beam would represent a measure ofthe X-position.

Instead of photo-diodes an array of optical fibres could be arrangedwith their ends in a straight line, akin to the technique of FIG. 4 butin only one direction. As with the just-mentioned photo-diode array,interception of the beam by any one optical fibre would stop theY-counter; the intercepting optical fibre would be a measure of theX-position.

It will be appreciated that the two straight edge members as shown inFIG. 6A could be modified so that each comprised a linear array ofphoto-diodes or optical fibres such that the X-Y positions would bemeasured by the intercept along each edge rather than by use of aseparate scale as illustrated in FIG. 6B. The use of one or more movablestraight edge members has the effect of simulating a matrix of detectorelements such as is illustrated in FIG. 4 while normally allowing thepassage of the laser beam. It will be realised that the fibre opticmatrix of FIG. 4 can be replaced by other elements such as a matrix ofphoto-diodes. The interception of the beam could be combined with areading of a position-representating voltage from a slide potentiometerin an arrangement similar to that discussed below with reference toFIGS. 8A-C.

The use of an apertured target plus the measurement of movement can berealized by an arrangement in which the target aperture is automaticallyadjustable to remain aligned with the beam. One such arrangement isshown in FIGS. 7A and 7B. FIG. 7 shows a structure 10 with a laser 20and receiver 30 defining a beam path 40 for monitoring movements of thestructure by means of apertured targets 400 mounted to the structure atvarious places along the beam path. FIG. 7A shows a view of a target 400on the side facing the laser 20. FIG. 7B is a vertical section throughthe target on the beam axis. The target comprises a frame 410 whichsupports a pair of plates. The first, larger, plate 420 (Y-plate) ismounted for vertical movement within the frame 410 and carries athreaded member 430 engaging with a threaded stud 432 that is rotatablysupported in frame 410 and coupled to a motor 440 for rotationtherealong to drive the plate up or down as seen in the figures. Theplate 420 in turn carries a smaller plate 450 (X-plate) running inguides of the plate 420 to be movable in an orthogonal direction. Theplate 450 carries a threaded member 452 engaging a threaded stud 454 atright angles to stud 432 coupled to a motor 456 for rotation thereby todrive the X-plate to the right or left as seen in FIG. 7A. The X-plate450 is centrally apertured and has mounted coaxially with the aperture afour segment photo-detector 460 whose quadrants are divided on the X andY axes. The photo-detector 460 is apertured at 462 to provide a smallaperture to pass the beam of the dimensions envisaged with targets 42 ofFIG. 1.

In operation, the arrangement of the four quadrant detector is such thatif the aperture 462 is not aligned with the beam axis 40, signals areobtained from the detector 460 dependent on the X and Y directionoffsets. These signals are applied in a servo loop (not shown) to drivethe motors 440 and 456 to maintain the aperture 462 aligned with thebeam axis. Movement information in the X and Y directions is obtained bya separate measurement arrangement for each axis, such as thatillustrated in FIG. 6B or FIG. 8A to be described. Consequently thetargets 400 of FIG. 7 are beam tracking. The receiver 30 is notessential to their operation but is desirable as a means of checkingthat the beam axis remains in its desired alignment.

The quadrant detector 460 provides essential on-off signals as the beamimpinges on a quandrant or not. It could be replaced by an aperturedversion of the detector described with reference to FIG. 5A enabling aproportional output to be obtained for the servo loops and a directmeasure of beam deflection without the other measurement aid.

Another target design for a beam tracking arrangement is illustrated inFIGS. 8A and 8B. It is related to the interception of the laser beam bya straight edge such as illustrated in FIGS. 6-6B. In FIGS. 8A and B, asupport frame 500 defines a relatively large rectangular aperture 502.As seen in FIG. 8B respective guides 504, 506 are formed in the upperand lower sides of the aperture 502 to locate the ends of a positionsensing bar 508 that has a longitudinal (vertical) slot 510 that has awidth that is just bridged by the cross-section of the laser beam 511. Across-section of the slot is seen in FIG. 8C. The bar 508 is made infact of two parts 512 and 514. Part 512 is of relatively transparentmaterial having a chamfered edge portion 530 defining one side of theslot so that one edge of the beam 511 striking the chamfered edge 530will cause light to be transmitted transversely through part 530 to alongitudinally extending optical fibre (light guide) 532 leading to aphoto-detector (not shown). The other part could be likewiseconstructed. In this case, the quiescent condition is with the beam onboth chamfered edges. Upon the beam leaving one edge as sensed by therelevant photodetector, the motor 522 drives bar 508 in the direction tofollow the beam to restore the quiescent condition.

More preferably the part 514 is of different construction to part 512 inorder to provide a measure of the beam's Y-position relative to thetarget. Thus the edge portion of part 514 bounding the slot 510 can be alinear array of detector elements (fibre-optic or photo-diode) of thekind already discussed. An alternative would be a linear version of thepoint-sensing photo-detector described with reference to FIG. 5A. Asregards tracking in the X-direction the situation is the same as for theedge of part 512. The detector elements, if such an array is used, areAND-gated for tracking purposes to provide an output as long as the beamimpinges on the slot-bounding edge of part 514.

The upper end (as seen in figures) of bar 508 has a first projectingportion 516 (FIG. 8B) that carries a threaded member 518 engaged with athreaded horizontal stud 520 rotatably supported and driven by areversible motor 522 to traverse bar 508 across the aperture 502 in theX-direction. The upper end of bar 508 also engages via a secondprojecting portion 524, e.g. a bifurcated portion, the wiper of a slidetype potentiometer 526 which provides a voltage pick-off at the wiperthat is a measure of the X-position of the slot 510 when a constantvoltage is applied across the potentiometer track. The motor 522 isenergized in one sense or the other in a control loop by the beam nolonger impinging on one or the other respectively of the slot edges 530so as to maintain the beam within slot 510. The control loop circuitsmay be mounted within frame 500.

If tracking in the Y as well as the X direction was required, it wouldbe possible to provide an orthogonal slot arrangement to maintain thebeam in a Y-axis slot.

In order that the target can be set up with the beam just overlappingonto both orthogonal assemblies, the bar defining the slot can have edgeportions as shown in FIG. 8A, and one of the parts 512 and 514 can bemounted to be adjustable in the transverse direction so as to adjust theslot width.

In the system of FIG. 2 the use of two laser beams to detect skewing ofcradle 22 was discussed. Another requirement that may arise is toinstall the laser transmitter unit at intervals for surveillance overrelatively short periods. In this case the unit should be installed on alater occasion so as to have the, or each, monitoring laser beamdefining precisely the same spatial datum as on the previous occasion.While the laser assembly of FIG. 2 might be used to that end FIG. 9shows a laser assembly that is particularly designed with theserequirements in mind.

In FIG. 9 a laser housing 600 contains either a double-ended laser ortwo lasers mounted rigidly back-to-back. One laser beam 602 is emittedat port 604. The other is split by beam splitters (not shown) alongmutually orthogonal axes (X-Y-Z) and emitted as respective orthogonalbeams 606, 608 610 that impinge on respective detector matrices 612, 614and 616 that are fixedly mounted. The primary laser beam 602 is used forsurveillance or monitoring as described. The three secondary beams havetheir positions monitored by the three matrix detectors and recorded inan analyzer unit 620. The matrix detectors may use a fibre optic orphoto-diode matrix such as already described with reference to FIGS. 4Aand 4B. After demounting the housing 600 after one period ofsurveillance, when it is installed for a further period the housing ispostioned to give the same X, Y and Z positions of the secondary beamsrecorded from the previous installation. Thus, provided the internallaser assembly within housing 600 is constructed to accurately set theaxis of primary beam 602 with respect to the secondary beams 606, 608and 610, the housing can be remounted with the beams aligned as beforedemounting. The secondary beams could be generated by separate lasersinstead of using beam splitting.

The apparatus described uses laser beams which are known for their lowdivergence and the beam can be focused by transmitter optics to convergeon a desired target point or the receiver. In order to still furtherimprove accuracy it is not proposed that a spatial filter can beemployed in the receiver. Such an arrangement is shown in FIG. 10A for afocused converging beam and in FIG. 10B for an unfocused diverging beam.

In FIG. 10A the laser source 20 and its optics 28 focus the beam 40 onthe detector 31 of the receiver 30. In front of the detector 31 ismounted an iris 33 (FIG. 10C) having an aperature 33A that effectivelyselects only the central axial portion of the beam for detection andthus acts as a spatial filter. FIG. 10B shows the even more pronouncedspatial filtering performed on a diverging beam 40' where the effectivedetected beam is reduced to the axial portion 40A. This spatialfiltering technique is of general utility.

Yet another monitoring system is illustrated in FIG. 11 and isparticularly intended to provide a relatively inexpensive system inwhich a plurality of targets can be used in line, each providing ameasure of the laser beam position relative to the target. Sucharrangements may be of advantage in civil engineering such as fortemporary or permanent monitoring of movement of embankments.

In FIG. 11, a laser 20 is mounted to its cradle supported on the groundof an embankment say. The laser beam 40 has a plurality of targets 700fixed to mounting posts in the ground along the section to be monitoredand preferably means 710 is provided to stop the beam, e.g. anon-reflective plate, at the end of the monitored section. Each targetis as shown in FIG. 11A. The target comprises a rectangular (square)frame 720 which carries across the aperture 722 a mesh 724 of finecross-wires of say 0.02 to 0.1 mm cross-section, the wires being spacedsay at 5 mm intervals. The mesh 724 is an open mesh of orthogonallyarranged wires forming an X-Y measurement grid within the frame. Thewires are bonded at the intersections and all the cross-points asexemplified by points 726 are made reflective by applying a reflectivecoating if necessary. The frame is conveniently marked with X-Y scales728, 729 in accord with the grid on the surface facing laser 22. Thetarget 700 is mounted to a support member 730 which in turn is attachedto a suitable ground post. Preferably as shown in the side view of FIG.11B the target is mounted in a double-sided shroud housing 732 toprotect it against the weather.

In operation the targets 700 are set up along the laser beam datum. Thebeam is in the visible spectrum to provide a visible indication of itsposition relative to each target by seeing the reflection at thecross-points as exemplified by the detail view of FIG. 11C of a mesh 724with the beam 40 illuminating two cross-points 726. The mesh dimensionsare chosen with regard to the fineness of measurement required and theneed as seen from FIG. 11C to allow sufficient beam to pass through themesh to illuminate at least one further target.

The system is relatively simply and inexpensive to install and theposition monitoring is performed by simple manual inspection. In someinstances it may be sufficient to note whether or not the beam remainson the mesh. For monitoring slippage of an embankment, say, beyondlimits set by the overall size of the mesh, the frame 720 bounding themesh on the side facing the laser could be provided with aphoto-detector arrangement providing a warning output if the beam startsto impinge on the frame.

In some circumstances, the path surveilled by the various systemsdescribed may lie vertically or substantially so. In most such instancesthe laser source will be at the lower end of the path, for examplesupported at ground level. Particularly where systems are to be activeover a prolonged period there is a risk of dirt or rain falling on thelaser optics to reduce performance. The same problems arise with using aprotective window.

FIG. 12 shows a laser housing designed to resist ingress of dirt etc.The left of the figure marked A shows the housing open for the laserlight. The right of the figure marked B shows the housing sealed. Thelaser 20 is mounted vertically in a closed weatherproof housing 800 theupper end of which is provided with a small aperture 802 through whichthe laser beam exits the housing 800. The aperture need only be small,just sufficient to pass the beam. The housing contains an air inlet 804,shown as being in the lower end of the housing, connected to acompressor or blower 806 which continuously blows a current of filteredair through the housing to leave at the aperture 802. Because theaperture is small a pressure well in excess of atmospheric is readilyachieved in the housing such that the air jet forced through aperture802 is sufficient to prevent water droplets, dust or the like fromentering the housing.

As well as having a protective function for the laser optics 28, the airflow along the laser assembly can be used for temperature regulation,for example for cooling.

It is also desirable that the laser and its optics should remainprotected if the air supply fails. To this end, the aperture 802 isprovided with an exterior closure flap 808 pivotally mounted at 810.Within the housing a pressure responsive resilient diaghragm 812 ismounted and is coupled to the flap through a link 814. In the absense ofsufficient pressure the resilient diaphragm closes the flap as seen atB. With the air supply operating as required the diaphragm causes theflap to open as seen at A.

Although the invention has been described mainly with reference todetecting movement of building structures, it can be applied to otherstructures such as plant and equipment susceptible to dynamic and/orother movement. Pressure vessels come to mind.

The preceding description has disclosed numerous ways of detectingrelative movement between a target and a laser beam datum. It will beappreciated that the same techniques can be applied to bring parts ofstructures or components or machinery into a required alignment byaligning a target on one part with a reference beam established onanother part.

Referring again to FIG. 6, the system there illustrated has a number oftargets mounted along a structure. FIGS. 6A and B show details of atarget construction having two orthogonally disposed movable membersproviding beam-intersecting straight edges. FIG. 6B shows an actuatorfor one such member and a measuring scale device provided between themember and a frame portion of the target.

In some circumstances where a relatively infrequent check of a structureis to be made, it is thought desirable to provide a system in which asmall number, and preferably one, target is used to monitor a structureat a number of points. In such cases it may also be sufficient to have atarget of the general kind disclosed above on which actuation of the oreach movable member is done manually and reading can be taken manually.Such a facility may be particularly useful in monitoring civilengineering structures such as embankments, banks of reservoirs anddams. One such system has already been described with reference to FIGS.11, 11A, 11B and 11C. Another such system having features more akin toFIGS. 6 to 6B is shown in FIGS. 13 to 13C.

In FIG. 13 an embankment or like work 350 has a plurality oftarget-mounting points 352 established along the portion of theembankment to be monitored. At fixed points 354 at either end of thepath away from the portion that may be subject to movement, a lasertransmitter 20 and receiver 30 are demountably set up, though with highrepeatability of positioning at each re-mounting. Preferably thereceiver is spatially filtered as described with reference to FIGS.10A-C.

As seen better in FIG. 13A, each target-mounting point 352 comprises asubstantial base 360, e.g. concrete, set in the ground to movetherewith, and capped by a support plate 362 secured to the base and towhich a tripod 364 is rigidly secured. The tripod in turn carries atarget-support plate 366 to which a target 370 is detachably mounted.The support plate 366 anda target 370 are constructed to have aprecisely repeatable mounting, i.e. provided with means accuratelylocating one with respect to the other.

Looking at use of the system in general, the tripods will be permanentlyavailable at the points 352. Whenever a periodic check is called for thelaser and receiver are re-established at 354 to establish the laser beam40 as a datum the same as on the previous measurement. The target isthen set up on the tripods in succession and a measure of the relativeposition of the beam to the target is made at each point 352. The valuesobtained can then be compared with those obtained on a previousoccasion. Thus no more than one target need be used, though more couldbe used if desired.

Referring to FIGS. 13B and C the target 370 has a frame 372 in which aresupported two members 374 and 376 providing straight edges 375 and 377respectively movable in orthogonal directions (X-Y). As better seen inFIG. 13C the movable member 374 is moved by coupling to the spindle 378of a micrometer 380 whose body 382 is fixed to the target frame. Theother member is likewise actuated. As already discussed in relation toFIG. 6, the relative beam position is measured at the point ofintersection of the beam by the straight edge. This can be signalled bymeans not shown from the beam interruption detected at the receiver 30or by providing a light guide at the straight edge cooperating with asuitable detector. As already described only one movable member isnecessary if its straight edge is provided with means--a linear array ofoptical fibre ends or of photo-diodes--to make a measurement in thetransverse direction. However the use of two manually adjustablestraight-edge members is simple requiring the minimum of additionalextra circuitry.

The signalling from the receiver can be done by means of a toneinterrupted when the beam is interrupted. To overcome possible jitter atthe point of intersection, an edge-triggered device could be used,triggered to interrupt the tone at a point of predetermined signalreduction. Alternatively, the tone could be in one state, say one, whenthe beam was not interrupted or fully blocked but off in theintermediate intersection zone, at which time the measurement would betaken.

It will be appreciated that the intersection point can be read off themicrometer scale as in FIG. 13C or a separate caliper-like scale couldbe provided on the frame 372 which is read against an index on themovable member. In this case the manual drive could be a simple leadscrew or other equivalent.

While the system described has tripod supports, these could be replacedby posts or any other suitable support. The supports themselves may bemade detachable from the base. Again there should be an accuraterepeatability of positioning when re-mounting the support to the base.

It will also be appreciated that the system could also be employed inre-aligning a structure subject to differential movement, where parts ofthe structure along the monitored path were capable of being adjusted inposition.

In the foregoing description the laser light used may be in the visiblespectrum or outside the visible spectrum except in those embodimentswhich specifically require viewing of the beam when obviously lightwithin the visible spectrum is used. The use of light within the visiblespectrum is generally of value in setting up equipment so that the beamcan be seen.

We claim:
 1. Apparatus for montoring deflections of a structure relativeto a laser beam, comprising:(a) a structure to be monitored; (b) a lasermounted at a first location to direct a laser beam along a pathextending adjacent a portion of the structure; (c) at least one targetmounted to said structure at a location adjacent the path of the laserbeam, wherein said target comprises: (d) a support portion fixed to thestructure at the target location, (e) at least one member having a beamintercepting edge movable with respect to said support portion betweenpositions displaced from and intercepting the beam, respectively, (f)actuator means acting between said support portion and said member tomove said member between said positions, (g) means for detecting theinterception of the beam by said member, and (h) means for measuring theposition of the member relative to the support portion at beaminterception.
 2. Apparatus as claimed in claim 1, wherein said targetcomprises:(a) a further member having a beam intercepting edge movablewith respect to said support portion between positions displaced fromand intercepting the beam, respectively, said one member and saidfurther member being movable in different directions; (b) furtheractuator means acting between said support portion and said furthermember to move said further member between said positions, and (c)further means for measuring the positions of said further memberrelative to the support portion at beam interception.
 3. Apparatus asclaimed in claim 1, wherein said beam intercepting edge and is movablealong an axis normal to said edge is a straight edge and to the laserbeam.
 4. Apparatus as claimed in claim 3, wherein said edge comprises aplurality of means spaced therealong to detect the interception of thelaser beam thereby, anad provide a measure of the beam position relativeto the target in the direction of said edge.
 5. Apparatus as claimed inclaim 1, wherein said interception detection means comprises a laserreceiver at a location remote from said laser and beyond said target. 6.Apparatus as claimed in claim 2, wherein the beam intercepting edges ofsaid one member and said further member are both straight edges, andsaid straight edges lie at right angles to one another, each of said onemember and said further member being individually movable in a directionnormal to its beam intercepting edge and to the laser beam.
 7. Apparatusas claimed in claim 6, wherein said interception detection meanscomprises a laser receiver at a location remote from said laser andbeyond said target.
 8. Apparatus as claimed in claim 32, wherein each ofsaid straight edges is provided with respective means for detecting theinterception of a laser beam thereby.
 9. Apparatus as claimed in claim1, wherein the actuator means is manually driven.
 10. Apparatus asclaimed in claim 1, wherein the actuator means comprises a motor drive.11. Apparatus as claimed in claim 10, comprising a plurality of targetssecured to said structure at spaced locations adjacent the beam path,and control means for successively actuating the actuating means foreach target such that successive targets provide measurements and aremaintained in a non-beam intercepting position when not being actuatedfor measurement.
 12. Apparatus as claimed in claim 1, wherein saidmember comprises two parallel, spaced apart beam intercepting edgesmovable together such that the beam passes between the edges, and beaminterception detecting means on each edge for detecting relativedeflectios of the beam toward one or another edge.
 13. Apparatus asclaimed in claim 12, wherein the beam interception detecting means onone edge comprises means for detecting the position of the beam alongsaid one edge.